This disclosure relates to the collection of charged particles in an electrically resistive relatively porous bed of fibers or other material. In comparison to other types of electrostatic precipitators, the present invention represents an improvement in terms of higher collection efficiency for small particles, much lower cost, and smaller equipment size. As compared to conventional filters, this represents a great improvement with respect to energy requirements associated with filter pressure loss, allowable operating velocity, and cost.
Several earlier studies have dealt with collection of charged particles by fiber beds, but did not demonstrate the dramatic influence from electric charge which is observed in the present invention. Prior researchers have developed a theoretical analysis of the deposition of particles on spheres and cylinders. This has directed those in the art to propose use of an electrically-charged filter made from fine wire. Others have tested fibrous filters consisting of layers of fibers separated by charged metal screens. Adjacent screens carried opposite charges, giving rise to an electric field which enhanced collection.
Research with respect to penetration of sub-micron particles in filter paper has included study of the results of an electric charge on the particles. The electric charge enhanced collection, but not remarkably so. Reported results for 0.3 micron particles showed penetration of highly-charged particles of 17% compared to 25% for uncharged particles at a linear velocity of 1.3 feed per minute. Charging of particles has also been demonstrated to be important in the collection of particles by fabric filters. Others have described a two-material filter designed to improve particle collection by building a charge on the filter by contacting it with a belt. The charges were observed to enhance the collection of atmospheric dust, but the enhancement was modest and the absolute removal efficiencies were too low to be of practical interest.
An article title "Effect of Particle Electrostatic Charge on Filtration by Fibrous Filters" by Lundgren and Whitby, I & EC Process Design and Development, Vol. 4, No. 4, October, 1965, reported experiments in which unipolar charged dye particles were captured in filters made from wool felt, urethane foam, and silver-plated glass fibers. The article concluded that "image forces;" (i.e. the force between a charged particle and its electrical image in a fiber) caused the observed enhancement of collection efficiencies when the particles were charged. Improvement in removal efficiency for sub-micron particles was typically modest and limited to low speed applications. As an example, 0.1 micron particles were captured within a polyurethane filter with an efficiency of 28% when charged with 6 electronic units, compared to an efficiency of 18% for neutral particles. The results were obtained with an air velocity of 10 feet per minute. Increases in the collection efficiency of a felt filter were obtained by charging the particles, but again the reported air flow velocity was only 4.4 feet per minute. These results are believed to demonstrate that image forces are not sufficiently strong to enhance collection of sub-micron particles in filter beds in a fast-moving air stream.
In the present invention, image forces are not the dominant means for collecting particles. This is demonstrated by an observed large decrease in collection efficiency when the bed was wet by water spray and by a great decrease in efficiency when using a conductive filter bed. In the case of a wetted or conductive bed, image forces would be expected to dominate the collection.